RAMALLAH, West Bank ? A U.S. mediator met with Yasser Arafat at his besieged headquarters Friday to renew truce efforts, and Israeli troops reportedly killed the mastermind of the Passover suicide bombing that killed 26 people and triggered its military offensive.

Israeli tanks entered new Palestinian territory in the weeklong offensive Tubas, a town of 20,000 in the West Bank and attack helicopters battled hundreds of gunmen in Nablus and nearby refugee camps. Smoke from burning cars and shops rose from downtown Nablus and frightened residents huddled in the innermost rooms of their apartments.

Two Palestinian militia leaders were killed, including alleged bombing mastermind Qeis Odwan, who headed Izzedine al Qassam, the military wing of the Islamic militant group Hamas, Hamas sources and Israel TV said. Odwan and five other members were killed when their hide-out in Tubas was shelled, sources and TV said. The army had no comment.

Izzedine al Qassam has carried out a series of suicide bombings in Israel, including the Passover attack on March 27 and a bombing in the northern port city of Haifa on March 31, in which 15 people were killed.

In Nablus, Nasser Awais, leader of the Al Aqsa Martyrs’ Brigade, was killed when explosives strapped to his body went off prematurely, said a militia spokesman, Abu Mujahed. The Al Aqsa militia is linked to Arafat’s Fatah movement.

More than 20 Palestinians, including several gunmen and a 14-year-old girl, were killed Friday. The Israeli military also retrieved the bodies of five men in Bethlehem, apparently killed by fellow Palestinians as suspected informers for Israel.

Israel’s offensive continued despite a tough speech by President Bush on Thursday in which he demanded that Israel halt its incursions and announced he would send Secretary of State Colin Powell to the region next week. Bush also accused Arafat of not confronting terrorists, and said his difficult situation was largely of his own making.

U.S. envoy Anthony Zinni met for 90 minutes Friday with Arafat, becoming the first senior official in eight days to meet with the Palestinian leader, who has been confined by Israel to a few rooms in his West Bank headquarters since last week.

Arafat adviser Nabil Abu Rdeneh said there would be more meetings between Palestinian and U.S. officials later in the day. Palestinian negotiator Saeb Erekat later said that Israel was preventing the follow-up talks, a claim Israel denied.

Arafat’s deputy, Mahmoud Abbas, said Arafat told Zinni that the Palestinians support a cease-fire deal negotiated last year by CIA chief George Tenet. Israel and the Palestinians have been at odds over the timetable for implementing the agreement.

Also Friday, an Israeli helicopter fired missiles on a car in Hebron driven by Ziyad Shuweiki of the Islamic Jihad militant group, but he escaped, witnesses said. They said five bystanders, including an 8-year-old boy, were injured. The Israeli military had no immediate comment.

Defying Bush’s demand that Israel halt its week-old offensive, Israeli Defense Minister Binyamin Ben-Eliezer said Friday that “we are continuing the operation we started.”

Israel has occupied six main Palestinian towns and several smaller ones and arrested hundreds of Palestinians.

In Bethlehem, a standoff between Israeli forces and about 240 gunmen holed up in the Church of the Nativity, built over the site where tradition says Jesus was born, entered a fourth day. Four of about 60 priests trapped in the church came out Friday and left Bethlehem under Israeli escort, the military said.

Giacomo Bini, a senior Roman Catholic official in Rome, said both Israel and the Palestinian gunmen have caused damage the shrine, one of Christianity’s holiest. Witnesses have said gunmen shot open a door when they forced their way into the church Tuesday, and the Palestinians have said Israeli forces blew open a door leading into the church courtyard Thursday.

In the Gaza Strip, some 10,000 supporters of Hamas rallied in the Jebaliya refugee camp. Sheik Ahmed Yassin, the Hamas founder, said the group would not stop attacks on Israelis, and he accused the United States of trying to weaken the Palestinians’ resolve by renewing truce efforts.

Also Friday, Israeli soldiers fired tear gas and stun grenades at journalists trying to cover the Zinni-Arafat meeting, and tanks pointed barrels at the group.

Israel had initially turned down Zinni’s request to see Arafat, but relented after Bush’s speech Thursday. Powell called Arafat early Friday, Abu Rdeneh said.

In Friday’s fighting, Israeli tanks rolled into Tubas, the first Israeli incursion since Bush demanded a halt to such raids. A 14-year-old Palestinian girl was killed by Israeli tank fire as she watched the scene from her balcony, Palestinian witnesses said.

The heaviest fighting was reported in the old city of the West Bank town of Nablus, the adjacent refugee camp of Balata and the refugee camp of Jenin to the north three strongholds of Palestinian gunmen.

Israeli helicopters and tanks fired shells and machine guns at the gunmen, who fired from behind barricades, witnesses said. Two people were killed in Jenin and nine in Nablus and Balata, witnesses and doctors said.

In Nablus, doctors opened a makeshift hospital in a mosque because ambulances could not enter, they said.

Noor Mansour, a resident of Jenin, said tanks and helicopters fired dozens of shells at the camp. “We can’t even look out the window,” Mansour said. “My family and I hid in a room in our house. We couldn’t move.”

On Thursday, four Israeli soldiers were killed, including three in intense fighting at the Jenin camp.

In a statement Friday, the Israeli military said it is holding 900 Palestinian prisoners and has confiscated 50 anti-tank grenades and two launchers, 26 machine guns, nine bombs, four belts for suicide bombers, dozens of boxes of ammunition, scores of kilograms of explosives, more than 1,300 rifles and more than 670 pistols.

Pressure was building on the Israelis to end their operation. The U.N. Security Council passed a resolution early Friday calling on Israel to withdraw “without delay.”

Israeli officials and newspaper editorials noted that Bush did not demand an immediate withdrawal from the West Bank and did not provide a timeline.

The Israeli Foreign Ministry welcomed the Bush initiative. “We note with satisfaction President Bush’s words on the necessity to put an end to terror,” the statement said.

The Palestinians also welcomed the Bush speech. In a statement, the Palestinian leadership said it accepted Bush’s declarations “without conditions” and criticized Israel’s West Bank campaign.

The military offensive is Israel’s largest in two decades. It included the call-up of about 30,000 reserve soldiers and a policy of “isolating” Arafat.

In southern Lebanon, authorities detained nine armed Palestinians, seized a missile and sent troops on border patrols in the strongest attempt yet to stave off cross-border fire exchanges between Israel and guerrillas, security officials said Friday.

However, the detentions did not target the Lebanese Hezbollah guerrillas, whose attacks this week on Israeli army outposts have provoked Israeli airstrikes and sparked fears of a new Mideast front.